Despite the fact the userbase has shrunk since removing user-created rooms, and the overwhelming amount of bots, Yahoo! Chat has been a location where many users go to converse with others, make new friends, and create communities with one another. I've seen many rooms with 50 users in them almost all day, which means people do still use this service.
To remove it outright would leave many of these users with no way to meet new people anymore, as Yahoo doesn't have a community otherwise. One of the biggest appeals to Yahoo as a messenger was the inclusion of chat -- otherwise, people could simply use AIM, or Skype, or just stick with only using Facebook or Google's services.
If we sign to convince Yahoo to retain their chat services, it keeps Yahoo! Messenger as an appealing messenger client, and people will continue to use it for the sake of meeting new people and making friends, and conversing with them all in chat.